In this work, we develop a methodology to explicitly obtain the modulation of wavepackets in shock-containing jets using the parabolised Floquet equations (PFE). This formulation was first developed by [1]} for the study of transition mechanisms in boundary layers and is applied for the first time in the present paper to study the coherent structures supported by imperfectly expanded jets. This formulation builds upon the spatially periodic framework developed in [2]}; here, the effect of the mean flow development can be directly estimated, providing a low-cost method for predicting the overall characteristics of the KH mode in these jets.
